Resistance welding apparatus with gun-arm mounted transformer

ABSTRACT

A resistance welding apparatus mounted on a robot arm includes a transformer having primary and secondary windings. Two gun arms supporting respective welding electrodes in confronting relation to each other are relatively movable toward and away from each other. Conductors extending from the secondary winding are electrically connected to one of the welding electrodes through one of the gun arms, and the transformer is supported on said one gun arm. Rectifiers are used which have terminals engaging said one gun arm and opposite terminals engaging the conductors. The rectifiers and the conductors are securely joined to the one gun arm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a resistance welding apparatus forsupplying a welding current between two electrodes mounted on respectivegun arms to weld workpieces clamped between the electrodes.

Resistance welding apparatus are widely employed to weld workpieces inshops and factories. A resistance welding apparatus has a pair ofelectrodes movable toward and away from each other. The electrodes whichclamp workpieces therebetween are supplied with a welding current tofuse and weld the workpieces to each other.

2. Description of Background Art

One resistance welding apparatus includes an apparatus housingfloatingly supported by a support bracket on which a transformer isfixedly mounted. The transformer has a secondary winding whose ends areconnected to ends of terminal plates. The other ends of the terminalsare electrically connected to two gun arms supporting respectiveelectrodes through flexible copper plates. The flexible copper plateshowever present a contact resistance which results in an electric powerloss and impose limitations on the direction in which the apparatushousing can be installed. The resistance welding apparatus is also heavyand large since the transformer is fixed to the support bracket throughdedicated holders.

There has been proposed a resistance welding apparatus which includes anapparatus housing floatingly supported by a support bracket and atransformer fixedly mounted on the apparatus housing, with one terminalplate of the transformer being directly coupled to one gun arm (seeJapanese Laid-Open Utility Model Publication No. 60(1985)-141977). Theproposed structure reduces any electric power loss and the overallweight of the resistance welding apparatus, and also allows theapparatus housing to be installed in any desired direction.

Efforts have been made in recent years to mount a resistance weldingapparatus on robot arms for automatizing welding operation. Therefore,there are demands for smaller and lighter resistance weldingapparatuses.

Other resistance welding apparatuses which find wise use today includefirst conductive plates connected respectively to the ends of thesecondary winding of the transformer, rectifiers and second conductiveplates attached to the first conductive plates in superposed fashion,and gun arms connected to the second conductive plates throughrespective flexible copper plates.

However, a large electric power loss is caused by joints between thesecond conductive plates connected to the rectifiers and the flexiblecopper plates and also between the flexible copper plates and the gunarms, and also by the internal resistance of the flexible copper plates.If a relative large welding current is required as when welding thickplates or plated steel plates in particular, then since a large electricpower loss would result from the contact resistance and the internalresistance, a large welding current cannot be supplied efficiently tothe electrodes.

At the time a relatively large current is supplied to weld thick platesor plated steel plates, the electrodes and the rectifiers themselves areheated to a considerably high temperature. Accordingly, various coolingmeans are employed to cool the electrodes and the rectifiers in order toavoid adverse effects which such a high temperature would have on theelectrodes and the rectifiers.

The cooling means are generally used exclusively to cool the electrodesor the rectifiers. Consequently, as many cooling means as the number ofthe electrodes and the rectifiers to be cooled must be employed inreality. Since such a number of cooling means need to be incorporated inthe resistance welding apparatus, the resistance welding apparatus islarge and heavy and highly costly.

In an attempt to reduce the size of a transformer in a resistancewelding apparatus, it has been proposed to use a toroidal core as atransformer core and also to design a secondary winding as a container.

FIG. 1 of the accompanying drawings shows a conventional transformer 2disclosed in Japanese Patent Publication No. 52(1977)-208, thetransformer 2 having a secondary winding doubling as a container. Thetransformer 2 includes a hollow outer conductor 4, a bottom conductor 8fixed to the lower end of the outer conductor 4, and a rodshaped innerconductor 10 extending upwardly from the center of the bottom conductor8 and serving as a terminal conductor. A toroidal core 14 with a primarywinding 12 mounted thereon is disposed around the inner conductor 10within the outer conductor 4. A cover conductor 18 is mounted on theupper end of the outer conductor 4 and has a hole 16 through which theinner conductor 10 extends. A rod-shaped terminal conductor 20 is fixedto the cover conductor 18. The cover conductor 20 has two holes (notshown) through which lead-out wires of the primary winding 12 extend outof the transformer container.

The electric circuit of the transformer 2 thus mechanically constructedis shown in FIG. 2. The primary winding 12 mounted on the toroidal core14 has X turns, and the secondary winding which is composed of the innerconductor 10, the bottom conductor 8, the outer conductor 4, and thecover conductor 18 has a single turn. Therefore, the transformer 2 has atransformation ratio of X : 1.

The rectifier circuit of the resistance welding apparatus is normally afull-wave rectifier circuit for higher efficiency and smaller ripple.The electric circuit of the transformer 2 shown in FIG. 2 may beassociated with a single-phase full-wave bridge rectifier circuit, asshown in FIG. 3, having rectifiers 22a through 22d connected in abridge. The single-phase full-wave bridge rectifier circuit however hastwice as many rectifiers as those of a singlephase full-wave center-taprectifier circuit. Accordingly, the rectifier circuit of the transformer2 is large in size and complex in construction.

The toroidal core 14 is made of silicon steel or ferrite. However, theloss caused by the toroidal core 14 is increased even if the transformer2 is employed in a resistance welding apparatus of the inverter type inwhich a direct current is converted to a high-frequency alternatingcurrent that is supplied to the transformer so as to be lowered involtage, and the alternating current is converted again back to a directcurrent by the rectifiers. The cross-sectional area of the toroidal corecannot therefore be reduced, and the transformer cannot be sufficientlyreduced in size and weight.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a resistancewelding apparatus which can reduce electric power loss and which islighter in weight and smaller in size.

Another object of the present invention is to provide a resistancewelding apparatus including a transformer having a secondary winding, apair of gun arms supporting respective welding electrodes in confrontingrelation to each other and movable toward and away from each other, anda conductor extending from said secondary winding and electricallyconnected to one of said welding electrodes through one of said gunarms, said transformer being supported on said one gun arm.

Still another object of the present invention is to further provide theresistance welding apparatus with a rectifier having a terminal engagingsaid one gun arm and an opposite terminal engaging said conductor, saidrectifier and said conductor being securely joined to said one gun arm.

Yet another object of the present invention is to further provide theresistance welding apparatus with a reinforcing member of anelectrically insulating material attached to said conductor, saidtransformer being supported on said one gun arm through said conductorand said reinforcing member.

Yet still another object of the present invention is to further providethe resistance welding apparatus with a robot arm, a bracket mounted onsaid robot arm, an actuator for displacing the other gun arm withrespect to said one gun arm, said transformer and said gun arms beingcombined as an assembly which is foatingly supported by said bracket.

A further object of the present invention is to provide a resistancewelding apparatus including a transformer having a secondary winding, aplurality of rectifiers connected to said secondary winding, a pair ofgun arms supporting respective welding electrodes in confrontingrelation to each other and movable toward and away from each other, andsaid rectifiers being held in direct engagement with one of said gunarms.

A still further object of the present invention is to construct theresistance welding apparatus wherein said rectifiers include first andsecond rectifiers having terminals held against respective sides of saidone gun arm, further including first and second conductors extendingfrom said secondary winding and held against the opposite terminals ofsaid first and second rectifiers, said first and second rectifiers andsaid first and second conductors being securely coupled to each other.

A yet further object of the present invention is to further provide theresistance welding apparatus with a robot arm, a bracket mounted on saidrobot arm, an actuator for displacing the other gun arm with respect tosaid one gun arm, said transformer and said gun arms being combined asan assembly which is floatingly supported by said bracket.

It is also an object of the present invention to provide a resistancewelding apparatus including a transformer having a secondary winding, arectifier connected to said secondary winding, a pair of gun armssupporting respective welding electrodes in confronting relation to eachother and movable toward and away from each other, said rectifier beingheld in direct engagement with one of said gun arms, and a coolantpassage defined in said one gun arm and extending near said weldingelectrode supported by said one gun arm and near said rectifier, forpassing a coolant therethrough to cool said electrode and saidrectifier.

Another object of the present invention is to further provide theresistance welding apparatus with a conductor extending from saidsecondary winding and held in direct engagement with said rectifier,said conductor having a coolant passage defined therein for passing acoolant therethrough to cool the interior of said transformer and saidrectifier.

Still another object of the present invention is to provide a resistancewelding apparatus including a transformer having a hollow conductivecontainer serving as a secondary winding, an annular core disposed insaid container, and a primary winding mounted on said annular core, arectifier having one terminal held against said container, an outputterminal connected to the other terminal of said rectifier, and a centertap mounted on an outer side wall of said container, whereby a weldingcurrent can be supplied from said output terminal and said center tap toworkpieces to be welded to each other.

Yet another object of the present invention is to construct theresistance welding apparatus above wherein said container is filled withinsulating oil.

Yet still another object of the present invention is to construct theresistance welding apparatus above wherein said container has a coolantpassage defined therein.

Yet another object of the present invention is to construct theresistance welding apparatus above wherein said core is made of anamorphous material.

A further object of the present invention is to provide a resistancewelding apparatus including a transformer having a hollow conductivecontainer serving as a secondary winding, an annular core disposed insaid container, and a primary winding mounted on said annular core,first and second inner conductors extending through the interior spaceof said core and electrically connected to said container, first andsecond flat conductors extending parallel to each other, said first andsecond inner conductors having ends embedded in confronting surfaces ofsaid first and second flat conductors, and extending in oppositedirections, first and second rectifiers having terminals held againstsaid first and second flat conductors, respectively, an output terminalconnected to the other terminals of said first and second rectifiers,and a center tap mounted on said container, whereby a welding currentcan be supplied from said output terminal and said center tap toworkpieces to be welded to each other.

Further scope of applicability of the present invention will becomeapparent form the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which a preferredembodiments of the present invention are shown by way of illustrationonly, and thus are not limitative of the present invention, and wherein:

FIGS. 1 through 3 show a conventional transformer;

FIG. 4 is an electric circuit of a resistance welding apparatusaccording to the present invention;

FIG. 5 is a perspective view of a transformer with rectifiers in theresistance welding apparatus shown in FIG. 4;

FIG. 6 is a side elevational view, partly in cross section, of thetransformer shown in FIG. 5;

FIG. 7 is a cross-sectional view taken along line VII--VII of FIG. 6;

FIG. 8 is an exploded perspective view of the transformer;

FIG. 9 is a perspective view of the transformer;

FIG. 10 is a cross-sectional view of the transformer;

FIG. 11 is an exploded perspective view of the transformer; and

FIG. 12 is a circuit diagram of the transformer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 4 schematically shows a resistance welding apparatus 30 accordingto the present invention. The resistance welding apparatus 30 basicallycomprises a converter unit 40, an inverter unit 42, and a transformerunit 44 with rectifiers.

Three-phase AC electric power supplied from a commercial three-phase ACpower supply 46 is converted to DC electric power by a rectifier stack48 and a capacitor 50 of the converter unit 40. The DC electric power isthen converted to AC electric power having a higher frequency, e.g., of10 KHz, than that of the three-phase AC electric power from the powersupply 46 by the inverter unit 42 of the full-bridge configuration thatis composed of transistors 52a through 52d.

The high-frequency AC electric power from the inverter unit 42 issupplied to a primary winding 58 of a transformer 56 of the transformerunit 44 through lead-out wires 57a, 57b connected to output terminals54a, 54b of the inverter unit 42. The primary winding 58 is wound on acore 60 made of an amorphous material. When the high-frequency ACelectric power is applied to the primary winding 58, an output voltageis induced across secondary windings 62a, 62b of the transformer 56between output terminals 63a, 63b thereof. The transformer 56 has acenter tap 62 between the secondary windings 62a, 62b. The outputvoltage from the transformer 56 is converted back to DC electric powerby rectifiers 64a, 64b of a single-phase full-wave center-tap rectifiercircuit, and the converted DC electric power is supplied to weldingelectrodes 66a, 66b between which workpieces Wa, Wb are clamped. Thewelding electrode 66a is connected to a common output terminal 68 of therectifiers 64a, 64b, whereas the welding electrode 66b is connected tothe center tap 62 between the secondary windings 62a, 62b.

As shown in FIGS. 5 through 7, the transformer unit 44, the weldingelectrodes 66a, 66b, and those members which are interposed therebetweenare supported as an assembly 78 by a support bracket 74 fixed to thedistal end of a robot arm 72 of a welding robot. The support bracket 74has an upper channel-shaped portion including two parallel legsextending downwardly and having respective tubular members 76a, 76b onlower ends thereof. The assembly 78 is floatingly supported by thesupport bracket 74 through the tubular members 76a, 76b.

The assembly 78 includes a pair of holder plates 80a, 80b fixedrespectively to the opposite ends of a cylinder 160, the holder plate80a having on its upper end an attachment 82 bent through about 90° andextending horizontally. Relatively long guide bars 84a, 84b extendthrough and are supported by the holder plates 80a, 80b. The guide bars84a, 84b are fitted in the respective tubular members 76a, 76b. Two coilsprings 86a, 86b are disposed around the guide bar 84a between theholder plates 80a, 80b and the tubular member 76a, and two coil springs86a, 86b are similarly disposed around the guide bar 84b between theholder plates 80a, 80b and the tubular member 76b.

A fixed gun arm 90 is attached to the attachment 82 of the holder plate80a through an insulation plate 88. The fixed gun arm 90 includes ahorizontal portion mounted at one end on the attachment 82 and adownward portion extending downwardly from the other end of thehorizontal portion. A welding electrode 66a is installed on the lowerend of the downward portion of the fixed gun arm 90. A planar outputterminal 68 (see FIGS. 8 and 9) is attached to said one end of thehorizontal portion of the fixed gun arm 90. The fixed gun arm 90 has acoolant passage 96 defined therein and extending to a position near thewelding electrode 66a, the coolant passage 96 having opposite endsopening outwardly in the output terminal 68. Nipples 98a, 98b areattached to the open ends of the coolant passage 96, and connected topipes 99a, 99b, respectively, extending from a coolant supply source(not shown).

As illustrated in FIGS. 10 and 11, the transformer 56 has a separablehollow cylindrical conductive container 100 in which the annular ortoroidal core 60 with the primary winding 58 wound thereon is disposed.Packings 102a through 102h of an insulating material are held againstthe opposite axial ends of the core 60. The packings 102a through 102hare supported by conductive support plates 104a, 104b of the container100. The container 100 has cooling water passageways 106a, 106b definedin its side wall and communicating with inlet ports 108a, 108b andoutlet ports 110a, 110b.

The side wall of the container 100 also has primary winding outlet holes116a, 116b fitted with tubular insulating members 114a, 114b,respectively. The lead-out wires 57a, 57b of the primary winding 58extend out through the outlet holes 116a, 116b, and are connected to theoutput terminals 54a, 54b of the inverter unit 42. One end of the centertap 62 is connected to a joint on the side wall of the container 100.

First and second flat conductors 120, 122 lying parallel to each otherare held against the respective support plates 102a, 102b throughrespective insulating plates 118a, 118b interposed therebetween. Thefirst and second conductors 120, 122 have disc regions 124a, 124b,respectively, and connector regions 126a, 126b, respectively, with firstand second inner joint conductors 128a, 128b having ends embedded in therespective disc regions 124a, 124b. The first inner conductor 128aextends through the support plate 104a and the hollow space in the core60 in insulated relation thereto, and engages the support plate 104b,thus electrically connecting the first conductor 120 and the supportplate 104b to each other. Likewise, the second inner conductor 128bextends through the support plate 104b and the hollow space in the core60 in insulated relation thereto, and engages the support plate 104a,thus electrically connecting the second conductor 122 and the supportplate 104a to each other.

The first and second conductors 120, 122 have inlet passages 130a, 130band outlet passages 132a, 132b communicating with the inlet ports 108a,108b and the outlet ports 110a, 110b, respectively, in the container100. The inlet passages 130a, 130b and the outlet passages 132a, 132bopen outwardly from the connector regions 126a, 126b, and nipples 134a,134b, 136a, 136b (FIGS. 6 and 8) are attached to the open ends of theinlet passages 130a, 130b and the outlet passages 132a, 132b. To thesenipples 134a, 134b, 136a 136b, there are connected pipes 138a, 138b,140a, 140b extending from the coolant supply source (not shown).

The container 100 has a hollow space 100a defined therein and filledwith insulating oil 141.

As shown in FIGS. 8 and 9, the transformer 56 is fixed to the outputterminal 68 of the fixed gun arm 90 through the rectifiers 64a, 64b.More specifically, the rectifiers 64a, 64b have respective anodes 142a,142b held directly against the opposite sides of the output terminal 68of the fixed gun arm 90, and respective cathodes 144a,144b engaging theconnector regions 126a, 126b of the first and second conductors 120, 122extending from the transformer 56. Insulating plates 146a, 146b areinterposed between the output terminal 68 and the connector regions126a, 126b and have respective openings 148a, 148b in which therectifiers 64a, 64b are disposed.

Presser plates 152a, 152b are held against the outer surfaces,respectively, of the connector regions 126a 126b with disc springs 150a,150b interposed therebetween. A plurality of bolts 154 are insertedthrough the presser plates 152a, 152b, the connector regions 126a, 126b,the insulating pates 146a, 146b, and the output terminal 68, and nuts156 are threaded over the threaded ends of the bolts 154, therebymounting the transformer 56 on the fixed gun arm 90 through therectifiers 64a, 64b.

If necessary, reinforcing members 158 made of an electrically insulatingresin material are integrally attached to the respective connectorregions 126a, 126b. The transformer 56 can be held more securely by theconnector regions 126a, 126b and the reinforcing members 158.

As shown in FIGS. 5 through 7, the cylinder 160 of the assembly 78 isdisposed between the holder plates 80a, 80b and has a piston rod 162extending toward the holder plate 80a. A movable gun arm 164 is securelyfixed to the piston rod 162. The welding electrode 66b is mounted on thedistal end of the movable gun arm 164 in confronting relation to theother welding electrode 66a. The movable gun arm 164 has a coolantpassage 166 defined therein and opposite outwardly open ends to whichrespective nipples 168a, 168b are attached.

An electric feeder 170 has one end connected to the movable gun arm 164.A flexible copper plate 172 has opposite ends joined to the other end ofthe electric feeder 170 and the center tap 62. A tubular insulatingmember 174 is disposed in the electric feeder 170, and the guide bar 84aextends through the tubular insulating member 174.

The resistance welding apparatus of the present invention is basicallyconstructed as described above. Operation and advantages of theresistance welding apparatus will be described below.

The areas of the two workpieces Wa, Wb to be welded together aresuperposed one on the other, and the welding robot (not shown) isoperated to cause the robot arm 72 to move the welding electrodes 66a,66b to the superposed areas of the workpieces Wa, Wb. At this time, thewelding electrodes 66a, 66b are spaced from each other by a distancecorresponding to the combined thickness of the workpieces Wa, Wb. Whenthe welding electrodes 66a, 66b have reached the areas of the workpiecesWa, Wb to be welded, the cylinder 160 is operated to displace the pistonrod 162 toward the workpiece Wb until the welding electrode 66b mountedon the movable gun arm 164 fixed to the piston rod 162 engages theworkpiece Wb.

The cylinder 160 is further operated, but the piston rod 162 isprevented from being displaced since the welding electrode 66b is heldagainst the workpiece Wb. Therefore, the assembly 78 floatinglysupported by the bracket 74 is moved in a direction away from theworkpiece Wb against the resilient forces of the coil springs 86a intothe position indicated by the two-dot-and-dash lines in FIG. 6. Sincethe fixed gun arm 90 is fixed to the assembly 78 through the attachment82 of the holder plate 80a, the assembly 78 and the fixed gun arm 90 aredisplaced in unison with each other away from the workpiece Wb. Thus,the other welding electrode 66a attached to the distal end of the fixedgun arm 90 is brought into engagement with the other workpiece Wa. Thewelding electrodes 66a, 66b now clamp the workpieces Wa, Wb therebetweenunder a predetermined force.

A primary power supply switch (not shown) of the resistance weldingapparatus 30 shown in FIG. 4 is turned on, and at the same time thecoolant supply source (not shown) is operated to supply a coolant suchas cooling water through the inlet passages 130a, 130b into the inletports 108a, 108b and the passageways 106a, 106b and also into thepassages 96, 166 in the fixed and movable gun arms 90, 164.

Three-phase AC electric power supplied from the three-phase AC powersupply 46 shown in FIG. 4 is converted to DC electric power by therectifier stack 48 of the converter unit 40, and the DC electric poweris applied to the inverter unit 42. The bases of the transistors 52athrough 52d connected in the full bridge configuration of the inverterunit 42 are supplied with pulse-width-modulated drive pulses from a basedriver circuit (not shown) for enabling the inverter unit 42 to generateAC electric power of a higher frequency, e.g., of 10 KHz, than thecommercial frequency of the three-phase AC power supply 46. Thegenerated AC electric power is then applied through the output terminals54a, 54b and the lead-out wires 57a, 57b to the primary winding 58 ofthe transformer 56. More specifically, as shown in FIGS. 10 and 12, thehigh-frequency AC electric power from the inverter unit 42 is applied tothe primary winding 58 which is wound as a predetermined number of turnsaround the toroidal core 60. The high-frequency AC electric powersupplied to the primary winding 58 then generates alternatinginterlinked fluxes φ which surround rectangular cross-sectional shapesprovided by the core 60.

The interlinked fluxes φ induce a secondary voltage across the secondarywindings 62a, 62b which are provided by the first and second innerconductors 128a, 128b and the container 100 including the support plates104a, 104b. The induced voltage is transmitted to the first and secondconductors 120, 122 through the first and second inner conductors 128a,128b. The AC electric power transmitted to the first and secondconductors 120, 122 is then rectified by the single-phase full-waverectifying circuit composed of the rectifiers 64a, 64b. The rectifiedcurrent is then supplied through the output terminal 68 and the centertap 62 between the welding electrodes 66a, 66b by which the workpiecesWa, Wb are clamped. The areas of the workpieces Wa Wb which are held incontact with each other are fused and joined to each other.

After the desired areas of the workpieces Wa, Wb have been welded toeach other, the cylinder 160 is operated to displace the piston rod 162back into the cylinder 160.

The welding electrodes 66a, 66b are now moved away from the workpiecesWa, Wb. Then, the robot arm 72 is actuated to move the weldingelectrodes 66a, 66b toward next areas of the workpieces Wa, Wb which areto be welded together.

With this embodiment, the transformer 56 is electrically connected tothe fixed gun arm 90 through the first and second conductors 120, 122which are connected to the secondary windings 62a, 62b and extend out ofthe container 100. The thin output terminal 68 is attached to the end ofthe fixed gun arm 90, and the anodes 142a, 142b of the rectifiers 64a,64b are held against the opposite sides of the output terminal 68. Thefirst and second conductors 120, 122 are held in engagement with thecathodes 144a, 144b of the rectifiers 64a, 64b. The output terminal 68,the rectifiers 64a, 64b, and the first and second conductors 120, 122are securely put together by the bolts 154 and the nuts 156 as shown inFIG. 9.

The above arrangement largely reduces the contact resistance between thetransformer 56 and the fixed gun arm 90, and hence any electric powerloss therebetween. Accordingly, efficient welding operation can beachieved by the welding electrodes 66a, 66b.

Furthermore, the transformer 56 is held with respect to the fixed gunarm 90 virtually by the first and second conductors 120, 122. Therefore,any holders which have heretofore been dedicated to the transformer arenot required. As a result, the number of parts used is reduced, and theresistance welding apparatus 30 is reduced in size and weight. The loadon the robot arm 72 is thus reduced, and the resistance weldingapparatus 30 can be moved quickly through the robot arm 72, with theconsequence that the welding operation effected by the resistancewelding apparatus 30 is made efficient.

The first and second conductors 120, 122 are associated, if necessary,with the reinforcing members 158 of an insulating resin material. Thetransformer 56 can thus be held more firmly with respect to the fixedgun arm 90, and no unwanted electric power loss is caused because thereinforcing members 158 are electrically insulating.

The output terminal 68 is disposed on the end of the fixed gun arm 90,and the rectifiers 64a, 64b have ends held in direct engagement with theoutput terminal 68 and the other ends engaging the first and secondconductors 120, 122 extending from the transformer 56. Inasmuch as therectifiers 64a, 64b are directly coupled to the fixed gun arm 90, it isnot necessary to place any conductive plates and flexible copper platesbetween the rectifiers 64a, 64b and the fixed gun arm 90. As aconsequence, the number of electric contacts between the transformer 56and the fixed gun arm 90 is reduced, and the contact resistance isgreatly reduced accordingly. When a large welding current is supplied tothe welding electrodes 66a, 66b, the electric power loss is minimizedfor efficient welding operation.

With the arrangement of the present invention, the output terminal 68 isdisposed on the end of the fixed gun arm 90, the rectifiers 64a, 64bdirectly engage the output terminal 68, and the the output terminal 68defines therein the passage 96 extending near the welding electrode 66a.By supplying a coolant such as cooling water to the passage 96 throughthe pipe 99a, the cooling water cools the welding electrode 66a and therectifiers 64a, 64b simultaneously, and thereafter is discharged out ofthe passage 96 through the pipe 99b. Therefore, the resistance weldingapparatus 30 is reduced in size and weight as a whole as compared withthe conventional resistance welding apparatus which employ dedicatedcooling means for cooling the welding electrodes and the rectifiers. Thecooling arrangement of the invention is quite simple since only thepassage 96 for passing the cooling water therethrough is provided as acooling means.

The first and second conductors 120, 122 which cooperate with the outputterminal 68 in sandwiching the rectifiers 64a, 64b define therein theinlet passages 130a130b and the outlet passages 132a, 132b. When thecooling water is supplied from the pipes 140a, 140b to the inletpassages 130a, 130b, the interior of the transformer 56 and therectifiers 64a, 64b are cooled by the supplied cooling water. Therectifiers 64a, 64b are thus prevented reliably from being adverselyaffected by heat.

The heat of iron loss and copper loss produced by the primary winding58, the core 60, the rectifiers 64a, 64b, and those parts whichconstitute the secondary windings, such as the first and second innerconductors 128a, 128b, i.e., the heat generated in the container 100, iseffectively radiated through the insulating oil 141 and the container100 into the cooling water flowing through the passageways 106a, 106b.

With the present invention, as described above, the conductors extendingfrom the secondary windings of the transformer are coupled to one of thegun arms directly or through the rectifiers, and the transformer is heldon said one gun arm through the conductors. Accordingly, the electricpower loss between the transformer and said one gun arm can be reducedas much as possible. Since the transformer is held by one of the gunarms, no dedicated transformer holders are necessary, and the resistancewelding apparatus is reduced in weight and size.

In addition, because the rectifiers connected to the secondary windingsof the transformer are directly coupled to the gun arm, no flexiblecopper plates are required for electric connection therebetween, and thecontact resistance between the transformer and the gun arm is reduced toa large degree. The electric power loss which would otherwise be causedby the flexible copper plates and the contact resistance is thereforeeliminated or minimized, and hence highly efficient welding operation isrendered possible especially when a large welding current is suppliedbetween the electrodes for welding workpieces.

The rectifiers are held in direct engagement with one of the gun armswith the electrodes attached, and said one gun arm defines therein thecoolant passage extending from a position near the electrodes to aposition near the rectifiers. By supplying a coolant such as coolingwater, thus, the electrodes and the rectifiers can simultaneously becooled. As a result, the resistance welding apparatus is small and lightin its entirety, and is also highly efficient in operation.

The secondary windings which double as the transformer container havethe center tap, and the flat rectifiers are securely fixed to thesecondary windings. Therefore, the number of the rectifiers used isreduced to half the number of rectifiers connected in a single-phasefull-wave bridge rectifier circuit. The core is made of an amorphousmaterial for use with high-frequency AC electric power, and thetransformer container is filled with insulating oil for heat exchangewith the exterior of the container. The core, the rectifiers, and thetransformer can therefore be cooled efficiently, and the resistancewelding apparatus is reduced in size. The cooling capability can furtherbe increased by passing cooling water through the container, so that theresistance welding apparatus may further be reduced in size.

Although certain preferred embodiments have been shown and described, itshould be understood that many changes and modifications may be madetherein without departing from the scope of the appended claims.

What is claimed is:
 1. A resistance welding apparatus comprising:atransformer having a secondary winding; a pair of gun arms supportingrespective welding electrodes in confronting relation to each other andmovable toward and away from each other; a conductor extending from saidsecondary winding and electrically connected to one of said weldingelectrodes through one of said gun arms, said conductor being joinedwith said one gun arm, said transformer being directly supported on saidone gun arm; a robot arm having a bracket mounted thereon; and anactuator for displacing the other one of said gun arms with respect tosaid one gun arm; said transformer and said gun arms being combined asan assembly which is floatingly supported by said bracket.
 2. Theresistance welding apparatus according to claim 1, further including arectifier having a terminal engaging said one gun arm and an oppositeterminal engaging said conductor, said rectifier and said conductorbeing securely joined to said one gun arm.
 3. The resistance weldingapparatus according to claim 1 or 2, further including a reinforcingmember of an electrically insulating material attached to saidconductor, said transformer being supported on said one gun arm throughsaid conductor and said reinforcing member.
 4. The resistance weldingapparatus according to claim 1, said pair of gun arms movable toward andaway from each other along an axis extending longitudinally from theother one of said gun arms.
 5. A resistance welding apparatuscomprising:a transformer having a secondary winding; a plurality ofrectifiers connected to said secondary winding; and a pair of gun armssupporting respective welding electrodes in confronting relation to eachother and movable toward and away from each other, and said rectifiersbeing held in direct engagement with one of said gun arms.
 6. Theresistance welding apparatus according to claim 5, said rectifiersincluding first and second rectifiers having respective first terminalsheld against respective sides of said one gun arm, further includingfirst and second conductors extending from said secondary winding andheld against respective second opposite terminals of said first andsecond rectifiers, said first and second rectifiers and said first andsecond conductors being securely coupled to each other.
 7. Theresistance welding apparatus according to claim 5 or 6, furtherincluding a robot arm, a bracket mounted on said robot arm, an actuatorfor displacing the other one of said gun arms with respect to said onegun arm, said transformer and said gun arms being combined as anassembly which is floatingly supported by said bracket.
 8. Theresistance welding apparatus according to claim 5, said pair of gun armsmovable toward and away from each other along an axis extendinglongitudinally from the other one of said gun arms.
 9. A resistancewelding apparatus comprising:a transformer having a secondary winding; arectifier connected to said secondary winding; a pair of gun armssupporting respective welding electrodes in confronting relation to eachother and movable toward and away from each other; and a first coolantpassage, housed within one of said gun arms and extending near saidwelding electrode supported by said one gun arm and near said rectifier,for passing a coolant therethrough to cool said electrode and saidrectifier, said rectifier being held in direct engagement with said onegun arm.
 10. The resistance welding apparatus according to claim 9,further including a conductor extending from said secondary winding andheld in direct engagement with said rectifier, said conductor having asecond coolant passage housed therein for passing a coolant therethroughto cool the interior of said transformer and said rectifier.
 11. Theresistance welding apparatus according to claim 9, said pair of gun armsmovable toward and away from each other along an axis extendinglongitudinally from the other one of said gun arms.
 12. A resistancewelding apparatus comprising:a pair of gun arms supporting respectivewelding electrodes in confronting relation to each other and movabletoward and away from each other; a transformer having a hollowconductive container serving as a secondary winding, an annular coredisposed in said container, and a primary winding mounted on saidannular core; a rectifier having a first terminal held directly againstsaid container; an output terminal extending from one of said gun armsand held directly against a second terminal of said rectifier; and acenter tap mounted on an outer side wall of said container, a weldingcurrent is supplied from said output terminal and said center tap toworkpieces to be welded to each other.
 13. The resistance weldingapparatus according to claim 12, said container is filled withinsulating oil.
 14. The resistance welding apparatus according to claim12, said container has a coolant passage housed therein.
 15. Theresistance welding apparatus according to claim 12, said annular core ismade of an amorphous material.
 16. The resistance welding apparatusaccording to claim 12, said pair of gun arms movable toward and awayfrom each other along an axis extending longitudinally from the otherone of said gun arms.
 17. A resistance welding apparatus comprising:atransformer having a hollow conductive container serving as a secondarywinding, an annular core disposed in said container, and a primarywinding mounted on said annular core further comprising first and secondinner conductors extending through an interior space of said annularcore and electrically connected to said container, andfirst and secondflat conductors extending parallel to each other exteriorly of saidhollow conductive container, said first and second inner conductorshaving ends embedded in confronting surfaces of said first and secondflat conductors, and extending in opposite directions; first and secondrectifiers having respective first terminals held directly against saidfirst and second flat conductors, respectively; and an output terminalconnected to respective second terminals of said first and secondrectifiers, said container including a center tap mounted thereon,welding current being supplied from said output terminal and said centertap to workpieces to be welded to each other.
 18. The resistance weldingapparatus according to claim 17, said container is filled withinsulating oil.
 19. The resistance welding apparatus according to claim17, said container has a coolant passage housed therein.
 20. Theresistance welding apparatus according to claim 17, said annular core ismade of an amorphous material.
 21. A resistance welding apparatuscomprising:first and second gun arms supporting respective weldingelectrodes in confronting relation to each other and movable toward andaway form each other along an axis extending longitudinally from saidfirst gun arm; transformer means, having a secondary winding and mounteddirectly on said second gun arm, including respective first and secondflat conductive members extending from opposite exterior portionstherefrom and coupled to said secondary winding; first and secondrectifiers having respective first terminals respectively mounteddirectly against said first and second flat conductive members; and anoutput terminal mounted directly against respective second terminals ofsaid first and second rectifiers, said transformer housed within acontainer which includes a center tap mounted thereon, welding currentbeing supplied from said output terminal and said center tap to saidrespective welding electrodes for welding workpieces together.
 22. Theresistance welding apparatus of claim 21, said output terminal extendingfrom said second gun arm.
 23. The resistance welding apparatus of claim22, said first and second rectifiers sandwiched between said outputterminal and respective ones of said first and second flat conductivemembers.
 24. The resistance welding apparatus of claim 23, each of saidfirst and second rectifiers of disc shape including a first disc surfacecorresponding to a cathode and serving as said first terminal mounteddirectly against respective ones of said first and second flatconductive members, and a second opposing disc surface corresponding toan anode and serving as said second terminal mounted directly againstrespective opposing surfaces of said output terminal.
 25. The resistancewelding apparatus of claim 21, said first gun arm movable and saidsecond gun arm mounted fixedly with respect to said first gun arm. 26.The resistance welding apparatus of claim 25, said second gun armincluding a coolant passage housed therein for passing coolant forcooling said second gun arm and said respective welding electrode. 27.The resistance welding apparatus of claim 21, further comprising:a robotarm; a bracket mounted on said robot arm; and an actuator for displacingsaid first gun arm with respect to said second gun arm, said transformermeans and said first and second gun arms combined as an assemblyfloatingly supported by said bracket.